Daily Politics and Economics comment. Co-Author of How to Lose a Referendum – the Definitive Story of Why Britain Voted for Brexit [http://bit.ly/2ryynRG]. Economics and Politics teacher in London. 7 years' Independent experience after 4 years in the state sector. Prior to that a management consultant. All opinions here are my own

The “setting by ability” debate flared up again on Wednesday when Nicky Morgan, the Secretary of State for Education, had to deny that plans were being made to force schools to teach pupils in separate ability groups for certain subjects in the Conservative Party manifesto. As the day progressed, the speculation became more feverish, as it was rumoured a school would not be allowed to be rated ‘outstanding’ by Ofsted if it didn’t use setting in certain subjects. Thinktanks, unions, Labour and the Lib Dems queued up to pour scorn on these as yet unconfirmed plans, and Morgan had to point out that twitter chat and journalistic speculation doesn’t amount to facts. What was lacking, as usual, was any sensible assessment of why setting by ability for certain subjects might be beneficial for young people.

Before I start – setting is where pupils are set by subject, so could be for instance in top set English and bottom set Maths – whereas ‘streaming’ is where children are in one set for everything, whatever the subject. Ed Miliband makes much of his comprehensive school background, but far less of the fact that he was in the top ‘stream’ for all subjects so was taught separately from many of the pupils he makes such a virtue of having been educated with. What we are talking about here is setting, and it is important to understand that whatever people say, there is no set answer on whether it works or not. It really does depend on how it is implemented.

Setting can be advantageous if it allows the brightest pupils to be stretched properly as they can be taught at a level and pace that achieves that. This is especially important in those subjects in which learning is like a ‘cake’, in which you get the foundation right and then build up and up, and where different pupils could be at different levels of that cake. Research has been published by Ofsted that indicated that 65% of children who performed highly in Maths and English at Primary school but then went into all-ability classes at secondary school didn’t get an A* or A at GCSE and 27% didn’t even get a B.

It can also be argued that setting benefits lower ability pupils because they too can be taught at a level and pace that suits them. It enables teaching assistants and other resources to be concentrated where it is really needed as well. But research to back up the success of this is more sparse, leading to the Education Endowment Foundation – which is a charity that evaluates teaching practices – suggests that setting benefits brighter children but at the expense of middle and lower-ability pupils.

This would explain why the reaction of Mary Bousted – of the Association of Teachers and Lecturers (ATL) – said on Wednesday that “If Nicky Morgan is committed to closing the gap for disadvantaged children, the last thing she should do is to divide children into ability sets and to use Ofsted to enforce this. This is educationally unjustifiable. The evidence is overwhelming that this practice holds back poor children, denying them access to an appropriately demanding curriculum.”

Ms Bousted is correct, that given poorer children are more likely to be in the middle and lower-ability sets, and setting has been found not to benefit those sets, the research on what happened in the past does suggest it is not the best way to close the gap for disadvantaged children.

Yet there are two issues with this argument. One is that the unions say very little about what would help the brightest poor children – who are held back when they are taught in mixed ability classes. Secondly what has often happened is that setting has been implemented in a way that disadvantages the lower ability sets. If you give those sets the least competent teachers and don’t allow movement between sets, then they become sink sets that destroy a student’s self-esteem and opportunities to improve.

But if a school properly resources the lower ability sets, with the better teachers and with the ability to move between sets frequently, and if a school encourages teachers to differentiate WITHIN a set (because even within a set there are different abilities) then setting can make a difference.

In a week when social mobility has been on the agenda, the knee jerk reaction of some of the educational establishment to a practice that can help bright students – particularly from poorer backgrounds, to achieve at a much higher level – thus attaining better results overall, on the basis of pure egalitarianism and ideology is a shame. No, I don’t agree that schools should be forced to set pupils – particularly if their results are excellent when teaching mixed ability groups (which is possible although much harder). But if setting is implemented well, then it is certainly not educationally unjustifiable.